If you are selling a Bumbo then try and get the harness attachment before you sell to make it more attractive to buyers. If you are buying a secondhand Bumbo (or have already bought one) you should still be able to get the harness from the manufacturer.
....... and remember - don't ever use a Bumbo or similar on a raised surface. If baby isn't strapped in they could fall out
http://www.dailymail.co.uk/news/article-2188714/Four-million-Bumbo-Baby-Seats-recalled-dozen-reports-skull-fractures-infants.html?ito=feeds-newsxml